1. Field of the Invention
This invention relates to zoom lenses, and more particularly to high range zoom lenses suited for still cameras, cine cameras and video cameras.
2. Description of the Prior Art
Recently, for TV cameras and for ordinary cameras, there have been increasing demands for zoon lenses of extended ranges. The prior known methods of extending the variation range of the focal length are to increase the total movement of the variator, to strengthen the refractive power of the variator, and to increase the number of relatively movable lens units constituting the variator. However, these methods have drawbacks that when the total length of the lens system is increased, and the aberrational problem becomes difficult to solve. For example, Japanese Laid-Open Patent Application No. Sho 53-34539 discloses a zoom lens comprising, from front to rear, a first lens unit of negative refractive power and a second lens unit of positive refractive power with the separation therebetween being varied to affect zooming, or a so-called 2-component zoom lens, wherein the second lens unit is divided into three parts of positive, negative and positive refractive powers respectively of which the negative part is moved in differential relation to the positive parts, thus achieving an increase in the variation range of the focal length. This method, however, increases the diameter of the first lens unit. Otherwise, when the first lens unit is moved forward to adjustment the wide angle end, the oblique pencil from the close object point would be partly blocked. The total length of the lens also tends to increase.
FIG. 1 schematically illustrates a thin lens paraxial arrangement of the conventional 2-component zoom lens, where reference numeral identifies a first lens unit of negative power, and II a second lens unit of positive power, moving in a way indicated by arrows between wide angle and telephoto ends W and T respectively of the zooming range. To extend the range, the total movement of each of the two lens units I and II may be increased as shown by dashed lines. The increase in the total movement, however, calls for an increase in the length of the zoom lens not only in the longitudinal direction but also in the lateral direction, making it difficult to reduce the bulk and size of the lens.
FIG. 2 illustrates a dislocation of the skew ray to the image point at the maximum height occurring when the first lens unit I of the 2-component zoom lens is moved forward to focus down to shorter object distances. This diagram is depicted when a diaphragm which coincides with the principal plane of the second lens unit II for the purpose of simplicity. As is obvious from FIG. 2, the diameter of the first lens unit I is determined at the front terminal end of the focusing range, or the position shown by the dashed line, in the wide angle end of the zooming range, as in most zoom lens designs. That is, if this focusing method is adopted, a surplus clear aperture must be set in the front or first lens I unit.
On the other hand, U.S. Pat. No. 4,256,371 to the assignee of the invention discloses the principle of construction of a zoom lens of extended range by moving lens groups beyond the terminal ends of the total movement of the ordinary zooming range. The present invention is to achieve a much-desired increase in the variation range of the focal length by utilizing that principle of construction.
A first object of the invention is to provide a zoom lens of extended range.
A second object of the present invention is to extend the ordinary variation range of the focal length toward the wide angle side, and, particularly to the variation range of the focal length when the minimum value is already shorter, to further shorten the minimum value.
A third object is to achieve a much-desired increase in the variation range of the focal length without increasing the diameter of the front lens unit I, or despite further reduction in the diameter.
Other objects will become apparent from the following description of embodiments of the invention.